1. Field of the Invention
The invention relates generally to a read/write head positioning system used in magnetic data storage devices, such as computer hard drive memories, and, more specifically, to a method and apparatus for marking and detecting particular servo wedges to reduce the effect of closure error.
2. Description of the Related Art
Hard disk drives are used in computer systems to provide primary storage for large amounts of information. Data in a hard disk drive is stored and retrieved by positioning a read/write transducer or xe2x80x9chead,xe2x80x9d mounted on an actuator, over concentrically located areas defining tracks on a surface of a magnetic disk. Multiple such heads and disk surfaces are common in a disk drive. During normal operation, each disk is rotated at a relatively high RPM, e.g., on the order of 4200-7200 RPM, and each transducer is repositioned to store and retrieve the data. The transducer is positioned by means of a servo control system.
In conventional disk drives, the servo control system positions the transducer by reading servo sectors or xe2x80x9cwedgesxe2x80x9d spaced-apart at regular intervals on each track to form a xe2x80x9cservo track.xe2x80x9d This is known as an embedded or sampled servo system, where the servo system reads each wedge to calculate a position error signal (PES) and provide a correction current to the actuator for maintaining (track following) the transducer over a track. The servo system is designed to have a compensator which operates at a particular bandwidth to respond to minute errors which occur from wedge-to wedge.
The servo tracks are written during manufacturing of the disk drive by a servo writer unit. The servo writer stores a pattern for the servo wedges on each track and writes the pattern of wedges at precisely timed intervals. Each wedge is numbered according to scheme of identification and wedges at the same circumferential position across the disk (e.g. like spokes in a wheel) have the same wedge number. Ideally, the servo track is written so that a line describing the path of a transducer following the center of each wedge would be a perfect circle. Unfortunately, such an ideal situation is not feasible, therefore the pattern of wedges may be written with small deviations from the perfect circle due to disk runout and vibration which can occur during servo track writing.
Conventionally, the servo writer performs a repeated series of operations which includes a mechanical movement of the actuator to a desired track position, followed by writing the servo track pattern. The operation is repeated for each servo track required, typically in the thousands. In order to reduce the time required for writing such thousands of servo tracks, the servo writer begins writing the pattern of wedges as soon as the actuator has settled in a stable position after movement. This means that the first wedge to be written on each track can vary from track to track.
Each servo track written in the above manner will xe2x80x9cfreezexe2x80x9d the systematic deviations referenced above as written-in deviations such that the servo control system is still generally able to track follow using the compensator designed for the disk drive. However, a problem may arise when the servo control system attempts to provide a correction after reading the first written wedge on a track, having just previously correcting for the last written wedge. In effect, any systematic behavior pattern is truncated after writing the last wedge on a track, repeating the behavior pattern starting with the first written wedge. Consequently, the servo control system may tend to overcorrect for a perceived PES arising from reading the first-written wedge. This overcorrection can lead to inadvertent encroachment of the write transducer over an adjacent track and data corruption. The phenomena of pattern deviation between the last-written wedge and first-written wedge on each track is known as xe2x80x9cclosure error.xe2x80x9d In some disk drives, the closure error can be large enough to cause a track to be retired and if a sufficient number of tracks are retired, rejection of the disk drive.
What is needed is a method and apparatus for disk drive servo control systems to maintain head position within acceptable limits in light of such closure error.
The aforementioned needs are satisfied in one aspect by a hard disk drive for storing information, the device comprising a magnetic disk having a plurality of servo wedges positioned in substantially circular patterns forming concentric servo tracks and wherein a marker is used to identify the location of the transition between the first and last servo wedges written on the servo track and a spindle which rotates the magnetic disk. In this aspect, the invention also comprises a transducer for writing data to and reading data from the rotating magnetic disk and an actuator coupled to the transducer wherein the actuator moves the transducer such that the transducer can be positioned over a selected region of the magnetic disk. The system also includes a control system that controls the positioning of the transducer over the magnetic disk by the actuator, wherein the control system receives signals from the transducer indicative of the transducer encountering the marker such that the control system is aware of the transition between the first and last written servo wedges of each concentric servo track on the magnetic disk such that the control system can thereby account for closure error occurring at the transition between the first and last servo wedge written on each concentric servo track while controlling the positioning of the transducer.
In this way, the control system of the hard disk drive can be advised of the transition between the first and the last written servo wedges during track following. As such, the algorithm used to ensure proper positioning of the transducer with respect to the servo track can be modified such that the discontinuity caused by closure error at the transition can be dealt with in a more efficient manner during subsequent read/write operations of the hard disk. In one aspect, the control system will moderate the response of transducer realignment and positioning following the encounter of a closure error by deploying a slow compensator. Preferably the compensator has a damping factor for providing a slow response.
In another aspect, the invention comprises a method for writing servo tracks on a disk surface wherein the method comprises forming a marker to identify the location of a transition between first and last formed wedges on the track.